New Braunfels, Texas: George P. Bush's campaign flyers in Texas declare that he's "standing beside our president" — the one who called his dad, Jeb, an embarrassment to his family and a pathetic person.

At a political forum outside San Antonio, another Republican candidate brags about his "bigly" wins over Democrats. Another hopeful in Houston, just days after a mass shooting at a Florida high school killed 17 people, sent voters a photo of herself holding an assault rifle — below the words "Kathaleen Wall stands with Trump."

Texas holds the nation's first 2018 primary elections Tuesday, and the campaign is providing a vivid exhibition of the Trump effect in GOP politics. Some races are playing out in a roadshow of one-upping emulation of the combative president, in which there's no such thing as cosying up too close or too ardently, regardless of his rough edges or low approval ratings nationwide.

"I'm Robert Stovall, and like President Trump, I realise the swamp is the problem," begins a campaign ad for Stovall, a San Antonio Republican Party leader now running for Congress. He stands in a literal swamp wearing a "Make America Great Again" hat and cocks a shotgun at the end.

It's like nothing seen before in Texas politics, even when a Texan was in the White House. Love of George W. Bush's style of big government conservatism wasn't a staple of campaigns here. A decade later, his nephew George P. Bush, the Texas land commissioner, is thanking President Donald Trump's son Donald Trump Jr. for endorsing him in his bid for re-election.

File image of US president Donald Trump. AP

Texas candidates aren't alone in courting Trump diehards who make up the GOP base — and who can be counted on to show up in typically low-turnout primary elections. Michigan Attorney General Bill Schuette, who is running for governor, has trumpeted Trump's endorsement, while an outside group accuses his GOP rival of "abandoning" the president. In Indiana, three Republicans running for Senate are all portraying themselves as his most steadfast ally.

The impression of a Trump White House at war with its enemies is stoking the atmosphere. "It's about he's our Republican president and if we don't stand together and we don't defend the party and conservative ideas, no one is," said Brendan Steinhauser, an Austin-based Republican strategist. A Quinnipiac University survey in late February put Trump's approval rating at 86 percent among Republicans but just 37 percent overall.

As Republicans brace for a difficult election this November — the president's party typically loses congressional seats in the midterm cycle — they could not have asked for a gentler warm-up than Texas, where the GOP holds such a commanding edge that most election-year drama dissolves once the primaries are done. Democrats haven't won a statewide race since 1994 and are not seen as favoured to flip any of six congressional seats opened up by GOP retirements on Capitol Hill.

Among those leaving is US Representative Lamar Smith, who in January lauded Trump's physical fitness and said Americans were better off getting their news straight from Trump than from the media. Some of the 18 Republicans trying to replace him don't stop there: Outsider candidate Mauro Garza says he is running because "Donald Trump said I could be here." Chip Roy, a former top aide to Republican US Senator Ted Cruz, opens rallies by rhetorically asking how much better citizens' lives were a year into a Trump presidency.

Congressional candidate Francisco Canseco cast the alternative in dark terms to one crowd in Comal County, which is sandwiched between Democratic-leaning San Antonio and liberal Austin and went more than 70 percent for Trump in 2016. "If Hillary had won last election, America would be gone. Our Republic as we know it would be lost. But we are blessed to have Donald Trump," he said.

It was just what Al Torres, an undecided 64-year-old retired federal worker wearing a CIA jacket, needed to hear. His friends blame the news media for not giving Trump a fair shake and want a candidate on board with his agenda. "That was important to hear somebody say that they're not going there and fighting him, but supporting him," Torres said.